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ICD-10-CM 2027 diagnosis code

T66Radiation sickness, unspecified

Not billable — use a more specific code

T66 is a header code, so it can't be reported by itself. Choose one of the 3 more specific codes beneath it.

Looking up by condition? See ICD-10 code for Irradiation effects.

Code unchanged since ICD-10-CM took effect on October 1, 2015. Source: official FY2027 ICD-10-CM release from CDC/NCHS and CMS. Page updated September 29, 2026. About our data

About T66

T66 is the ICD-10-CM diagnosis code for radiation sickness, unspecified. It belongs to block T66-T78 (other and unspecified effects of external causes) and chapter 19 (injury, poisoning and certain other consequences of external causes). It is a header code, so it can't be reported by itself; one of its 3 subcodes must be used instead.

To report this condition, pick the subcode that matches the documentation: T66.XXXA (initial encounter), T66.XXXD (subsequent encounter) and T66.XXXS (sequela).

Don't report T66 together with the conditions in its Excludes1 note: burns (T20-T31), leukemia (C91-C95), radiation gastroenteritis and colitis (K52.0), radiation pneumonitis (J70.0) and 2 more.

It has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.

Specific codes under T66

Coding notes

Excludes1Not coded here — never report together with this code
  • specified adverse effects of radiation, such as:
  • burns (T20-T31)
  • leukemia (C91-C95)
  • radiation gastroenteritis and colitis (K52.0)
  • radiation pneumonitis (J70.0)
  • radiation related disorders of the skin and subcutaneous tissue (L55-L59)
  • sunburn (L55.-)

Notes that apply from higher levels

Instructions written at a parent level also apply to T66.

Broader instructions also apply from T66-T78 Other and unspecified effects of external causes and Chapter 19: Injury, poisoning and certain other consequences of external causes.

Alphabetic index entries

7 entries

Terms in the official ICD-10-CM index that lead to T66.

Codes whose notes reference T66

Code history

  1. 2016
  2. 2017
  3. 2018
  4. 2019
  5. 2020
  6. 2021
  7. 2022
  8. 2023
  9. 2024
  10. 2025
  11. 2026
  12. 2027
  • FY2016 (effective 10/1/2015): Added (first year of ICD-10-CM)
  • No changes since FY2016.

Common questions about T66

Is T66 billable?

No. T66 is a header code. Report one of its more specific subcodes instead.